The photopolymerization of unsaturated compounds can be substantially accelerated by a wide variety of initiators, including such compounds as acetophenone, propiophenone, benzophenone, xanthone, thioxanthone, fluorenone, benzaldehyde, anthraquinone, carbazole, thioindigoid dyes, various derivatives of these compounds and certain metalic salts such as ferric salts of carboxylic acids. Most of these initiators are water-insoluble and are not suitable for use in water-based systems. When a water-insoluble initiator is used in a water-based system it is necessary that the initiatior be dissolved in a non-aqueous solvent and dispersed with photopolymerizable material into the aqueous phase to form an emulsion. If the photopolymerizable material present is water-soluble, then the initiator is in a separate phase and cannot work very effectively to bring about photopolymerization on exposure to actinic light. If, on the other hand, the unsaturated photopolymerizable compounds are in the same (solvent) phase as the initiator, then although photopolymerization can take place more efficiently, the ability of the photopolymerized material to subsequently insolubilize the ingredients of the aqueous phase and in particular the protective colloid which is normally present is less effective.
In order to produce the most effective insolubilization of the aqueous phase ingredients by photopolymerization of the unsaturated compounds, it is preferable for the photopolymerizable unsaturated compounds to be totally or substantially water-soluble and for the initiator also to be water-soluble. When the initiator and unsaturated photopolymerizable material are both fully, or substantially, in the same (aqueous) phase, then photopolymerization on exposure to UV light can proceed efficiently and the resulting insolubilisation of the aqueous layer ingredients is effective.
For a photoinitiator to work efficiently when exposed to actinic light through glass (as is normally the case when the exposure is carried out with a printing down frame in screen printing), it is highly desirable for it to absorb above 3250 .ANG.. However, many simple water soluble photoinitiators do not absorb above this level. Moreover, is the solubilization of a photoinitiator is attempted by simple sulphonation or inclusion of a carboxy group, the presence of the carboxy or sulpho group on a main benzene ring of the initiator, whether directly attached or separated by a single methylene group, deactivates the molecule and makes it less efficient as a photoinitiator.
At present, the water-soluble photoinitiators used in commercial screen printing, as described in U.K. Pat. No. 2,108,979, are ferric salts of carboxylic acids (U.K. Pat. No. 1,307,995 and solubilized indigoid vat colors (U.K. Pat. Nos. 1,307,995 and 1,366,304).
The use of ferric salts of carboxylic acids requires the presence of hydrogen peroxide, which is at times undesirable.
The commonly used indigoid vat colors are thioindigoid dyes sold under the name "ANTHRASOL" by Farbwerke Hoechst. These are capable of photoinitiating the polymerization of unsaturated compounds on exposure to UV light. However, they suffer from the extremely serious drawback that, to remain active, the layer to be photopolymerized must contain moisture. This requires the inclusion of quite large quantities of humectant in the layer which, under conditions of high ambient temperature and humidity can cause reticulation and breakdown problems in the layer.
Another group of water-soluble photoinitiators, reported in U.K. Pat. No. 2,108,487, are thioxanthones of the formula ##STR1## wherein R.sub.1 is alkyl, alkoxy, alkylthio, halogeno, nitro, amino, alkyl-amino, di-alkyl-amino, hydroxy-alkyl-amino, alkanoylamino, benzoylamino, N-alkanoyl-N-benzoyl-amino, sulphonamido, or acetyl, R.sub.2 is alkylene, A is --COOH, --SO.sub.3 H, --OSO.sub.3 H, or --O--CO--X--COOH (where X is such that HO--CO--X--COOH is a di- or tri-carboxylic acid of up to 9 carbon atoms), n is 0, 1 or 2 and m is 1 or 2, provided that when A is --COOH, m is 2, the aforesaid alkyl, alkoxy, alkanoyl, and alkylene residues containing up to 4 carbon atoms each, as the free acid or as a salt thereof. Especially valuable compounds are those in which R.sub.1 is methyl, R.sub.2 is ethylene, trimethylene, or tetramethylene, A is --SO.sub.3 H, n is 0, 1 or 2 and m is 1 or 2 as the free acid or as a water-soluble salt thereof.
These thioxanthones, like the other water-soluble photoinitiators are salts which are sensitive to changes in pH, and the salt forming groups interfere with the stability of dispersions of collods, e.g. gelatin, which may be present in the polymerization system, resulting in the breakup of the stable colloidal dispersions. Besides, the presence of calcium ions in water, as occurs in hard water, may result in the precipitation of the calcium salts of the photoinitiators, thereby removing them from the system.